Typically a DMFC fuel cell is designed to operate at specific air and fuel flows which correspond to the electrical charge transferred to a connected load. In both stationary and portable power applications fuel cell equipment is required to operate for varying time periods and at different load levels and are subjected to frequent start-up and shut-down cycles.
In normal operation the fuel cell stack is designed to operate optimally and efficiently at a pre-determined power. Operating conditions are normally trimmed to the nominal power output and the fuel cell stack does not operate optimally in extremes of low-power or near shut-down conditions. A characteristic consequence of low-power operation is the rising relative loss by cross-over diffusion of methanol fuel. Deleterious effects of cross-over diffusion in a dormant fuel-cell and its detrimental effects on re-starting from the dormant stage are described by Odgaard (Published U.S. Application 2010/0310954). In particular concentration of methanol due to partial freezing of the water constituent of the mixture can lead to corrosion of the membrane electrode assembly (MEA) and local overheating on restarting. Odgaard (Published U.S. Application 2010/0310954) teaches the use of antifreeze additions to the cell when it is shut-down in order to alleviate these start-up problems.
Difficulties in enabling a balanced and stable mode of operation in a DMFC is related to the ability to quantify accurately the methanol concentration in the single cells and in the stack as a whole.
An external, non-intrusive system and method to monitor and control methanol concentrations via electrochemical impedance spectroscopy (EIS) in an operating DMFC system has been described by Odgaard and Yde-Andersen (Published U.S. Application 2009/0269625. The method assesses the cell operating condition, and in particular the methanol concentration, during full operation without or with minimal interruption. The results obtained from the alternating current response measurements may be optionally combined or supplemented by direct voltage measurement techniques. The measurement techniques are applied in order to control and adjust the methanol concentration in the cell by either adding pure methanol or pure water to the fuel compartment. Maintenance of fuel cell efficiency depends on the ability to maintain a correct concentration of the methanol in the fuel compartment on the anode side of the membrane, reducing and compensating for unavoidable loss and on a low internal resistance in the cell. Methanol diffusion through the electrolyte membrane causes a phenomenon known as cross-over of fuel. Methanol which reaches the cathode reacts wastefully with oxygen and does not produce electrons which thus do not traverse the external electrical circuit and cannot provide useful electrical energy. This situation is aggravated when methanol concentrations in the anode fuel compartment are raised because a high methanol concentration is a driving force for diffusion of methanol through the membrane. On the other hand, maintenance of the electrochemical reaction rates depends on the supply of adequate methanol. Denudation of the methanol concentration leads to reduced power generation. In a DMFC stack, the fuel is circulated through the stack and deplenished fuel is returned to the fuel compartment. Since part of the methanol is used by the electrochemical reaction, the methanol concentration in the compartment is reduced. Consequently, the individual cell and whole stack impedance will change unless the methanol concentration is maintained. The methanol concentration increases when water is lost thereby resulting in impedance increase. It is therefore desirable to control methanol concentration in fuel cells in order to optimize efficiency of the DMFC stack and maintain output. Satisfactory control can be achieved by measuring the methanol concentration and compensating for methanol consumption. Consumption of fuel can be calculated on the basis of the electrical charge transferred. The methanol concentration can be maintained at a specified level by addition of water as diluent or addition of alcohol as a concentrate or as a pure substance. Water and methanol can evaporate from the fuel tank, thereby affecting the methanol concentration. These concentration changes can be significant and may cause large deviations from the ideal alcohol concentration.
However, it is difficult to maintain stable operation at low power, and stack performance is unreliable at low power levels, making it impossible to operate DMFC fuel cells at variable power levels. Normal practice has been either to operate at nominal full-power output or alternatively to shut-down completely.
In addition to the problems of operation at reduced power output, it is necessary to protect the cell against transient variations in methanol fuel concentration and methanol diffusion through the electrolyte membrane when approaching and establishing shut-down status. Conditions at shut-down and afterwards determine the condition of the cells and stack on resumption of operation, and thus the ease of re-starting normal operation.
Shut down is affected by terminating the supply of oxidizing air to the DMFC by shutting off the air pump. The concentration of methanol fuel at the electrode determines how the cell will survive the interruption of air supply and the ease of subsequent restart.
Further, during cold periods it is necessary to establish a means of protecting the stack and individual cells from the effects of freezing. Odgaard (Published U.S. Patent Application 2010/0310954 discloses systems and methods for protecting fuel cell systems from frost by introduction of a freezing point depressant into the fuel cell system and/or flushing the fuel cell system with an insert gas and describes further problems arising due to the reactivity of methanol even though it is a material that can depress the freezing point of water sufficiently to protect a DFMC when stored in freezing conditions. In this published patent application, methods and systems for adding to a fuel cell system a freezing point depressant that is compatible with fuel cell material components and that does not deleteriously affect electrode processes of the fuel cell system are described as well as methods and system which utilize an inert gas, preferably carbon dioxide already present in the system as a reaction product generated upon the oxidation of methanol, to flush the fuel cell system during fuel-cell-shut-down. The published patent application further provides fuel cell deactivation processes that leaves the direct methanol fuel cell (DMFC) in a non-reactive state during dormant periods and provides for carefully controlled re-activation of the dormant cell without adding complexity and without lowering efficiency of the fuel cell.